Pumps are already known, having a movable piston defining a chamber of variable volume inside a pump body and actuating means formed by a push button which reciprocates the piston to increase the volume of the chamber whilst it is put in communication with a body of liquid and then to reduce the volume of the chamber whilst it is put in communication with the atmosphere, generally through of a spray nozzle. These are the respective phases of intake and discharge of the liquid. Generally the piston is moved manually in one direction away from the rest position and a return spring ensures its return to the rest position.
The known pumps have the drawback that the discharge of the liquid is dependent on gentleness or otherwise of the action of the user. This results in a lack of efficiency of the pump, in particular a poor quality of the spraying of the liquid because the behaviour of the spray nozzle is closely related to the discharge pressure.